In conventional communications systems, such as Wideband Code Division Multiple Access (WCDMA) systems, the transmission power may be adjusted by the receiving system requesting an increase in transmission power from the transmitting system. Thus, for example, a mobile terminal may request that a base station increase its transmission power if the signal-to-interference ratio (SIR) of the received signal is below a predefined threshold. As used herein, the term “mobile terminal” encompasses a wide variety of portable wireless devices that can access a cellular system. Mobile terminals include, but are not restricted to, a cellular radiotelephone with or without a multi-line display, a Personal Communications System (PCS) terminal that may combine a cellular radiotelephone with data processing, facsimile and/or data communications capabilities, a Personal Digital Assistant (PDA) that can include a radiotelephone, pager, Internet/intranet access, Web browser, organizer, calendar and/or a Global Positioning System (GPS) receiver, and conventional laptop, palmtop and/or pervasive computing devices that include wireless receivers.
In many communication systems, such as a WCDMA system, transmitted data is organized such that a number of data bits are collected in a transport block. Several transport blocks are, typically, contained in a transmission time interval (TTI). Typically, in order to decode the transmitted data in the transport blocks the complete TTI must be received. Cyclic redundancy check (CRC) bits are, typically, added on each transport block. The CRC bits have special coding properties such that if the CRC bits are found to be wrong, the bits in the transport block are almost certainly decoded incorrectly. Similarly, if the CRC bits are found to be correct, the bits in the transport block are almost certainly decoded correctly. The block error rate (BLER) for a transport channel may, therefore, be estimated by filtering CRC error flags (CRCef) of the decoded transport blocks.
In a conventional power control system for WCDMA uplink or downlink, an inner loop and an outer loop of the power control system are provided. The outer loop sets a reference SIR value based on the deviation of a measured BLER from a reference BLER. The inner loop compares the measured SIR to the reference SIR. If the reference SIR is below the measured SIR a request is made to increase the received power, and vice versa. In a typical two loop power control system, the inner loop may be updated at about 1500 Hz and the outer loop updated between about 10 and about 100 Hz.
While requests for changes in power may be made, there is typically no guarantee that the requested changes are carried out. For example, in a congested system, the base station might not be able to grant the requested change in transmitted power. If the outer loop power control system bases the measured BLER on the history of the received CRC error flags, the measured BLER could reach very high levels if the requested power is denied for a prolonged period of time. This increase in measured BLER may result in an increase in the reference SIR value such that the reference SIR may increase without bounds.
One potential solution to such an unbounded increase in the reference SIR value is to bound the absolute value. However, in practice it may be difficult to find a narrow enough absolute range within which the reference SIR value is allowed to operate.
In addition to increases in the reference SIR value when requests for increased power are not granted, when the base station finally grants the requested increase in power the measured BLER may be very high. If a proportional integral derivative (PID) controller is utilized as the outer loop power controller, the reference SIR value could increase at a rate larger than the time to observe the change in the measured BLER as a result of increases in power from the base station. As a result, an unnecessary amount of power may be requested from the base station.